1. Field of the Invention
The present invention relates to a humidifying device for fuel cells, to a method for humidifying a fuel cell membrane, and to a fuel cell.
2. Description of the Related Art
Current is generated in a fuel cell by a chemical reaction. In the process, fuel and oxygen are converted into electrical energy and water as the reaction product. A fuel cell or PEM fuel cell substantially comprises an anode, a membrane and a cathode, which together are known as the membrane electrode assembly or MEA. The membrane consists of porous, electrically conductive material and is arranged between the anode and the cathode in order to exchange ions. A fuel, such as for example hydrogen or methanol, is supplied on the side of the anode, while oxygen or air is supplied on the side of the cathode. Protons or hydrogen ions are generated by catalytic reactions at the anode, and move through the membrane to the cathode. At the cathode, the hydrogen ions react with the oxygen, and water is formed.
The reaction at the electrodes is as follows:
Anode: H2xe2x86x922H++2exe2x88x92
Cathode: {fraction (1/2 )}O2+2H++2exe2x88x92xe2x86x92H2O
Thereby, current is generated at the electrodes and is fed to a consumer.
Fuel cells of this type are known from a wide range of publications. However, the problem exists that the membrane or the MEA has to be kept moist.
If the membrane were to dry out, it would lose its ion conductivity and the fuel cell would no longer be able to function.
Therefore, it is proposed in U.S. Pat. No. 5,432,020 to add finely atomized water to the gas flow to the fuel cell by means of an injection nozzle. As a result, the membrane is cooled and kept moist.
However, with this arrangement the problem exists that it is necessary to carry along a supply of water. The supply of water takes up space and requires further measures, such as for example protection against freezing, which entails additional costs. Therefore, a need exists for a fuel cell having a membrane that remains moist without having the above noted associated problems.
Therefore, the object of the present invention is to provide a humidifying device for fuel cells and to describe a method for humidifying a fuel cell membrane, in which effective humidifying of the fuel cell membrane takes place inexpensively and in a space-saving manner. Furthermore, it is intended to provide a fuel cell which is inexpensive and the membrane of which is reliably kept moist.
This object is achieved by the humidifying device for a fuel cell as claimed in patent claim 1, the method for humidifying a fuel cell membrane as claimed in patent claim 6 and by the fuel cell as claimed in patent claim 12. Further advantageous features, aspects and details of the invention will emerge from the dependent claims, the description and the drawing.
The humidifying device according to the invention for a fuel cell comprises a membrane, which is coupled to the cathode exhaust gas of a fuel cell, the cathode exhaust gas being situated on one side of the membrane, and a connecting element which acts as a line connecting the other side of the membrane to the cathode or anode gas supply, so that in operation water vapor penetrates through the membrane, in order to humidify the gas, for example outside air or oxygen, which is fed to the anode or cathode. The humidifying device allows reliable humidifying of the membrane electrode unit of a fuel cell and can be produced in compact form and at low cost. It is therefore particularly suitable for series production.
The humidifying device advantageously comprises a vacuum pump for maintaining a concentration gradient of the humidity between the two sides of the membrane. This results in particularly effective exchange of the humidity through the membrane.
Furthermore, it is possible to provide a vessel which is used to receive and/or temporarily store water vapor or water.
It is preferable to provide a metering unit to humidify the gas flow which is passed to the cathode. This allows metering with the quantity of water required in each case as a function of the operating state of the fuel cell to take place.
The metering unit is advantageously connected to a control circuit in which the humidity of the cathode feed air or the humidity of the gas which is guided to the cathode is measured. This allows particularly precise metering to take place.
In the method according to the invention for humidifying a fuel cell membrane, the cathode exhaust gas of a fuel cell is connected, via a membrane, to an anode or cathode gas supply, for example outside air or oxygen, in order, by means of a concentration gradient of the humidity on the two sides of the membrane, for water vapor to be transferred from the cathode exhaust gas to the gas or cathode gas supply. Using the permeation of water molecules through the membrane allows water vapor to be transported without an external drive or without the additional use of energy and without the cathode exhaust gas being mixed with the gas supplied to the fuel cell.
The passage of water vapor through the membrane is preferably assisted by actuation of a vacuum pump. Generation of a vacuum allows the concentration gradient to be maintained. The result is a particularly high and uniform transfer rate for the water vapor.
The concentration gradient can also be maintained by constantly exchanging gas on one side of the membrane. This has the advantage that no pump is required. This solution is particularly inexpensive since components are saved and the outlay on energy in operation is reduced.
Preferably, humidity is temporarily stored in a vessel or water tank and can then be transferred in an accurately metered manner to the cathode gas supply.
A fuel cell according to the invention has a humidifying device according to the invention.